This invention relates to negative-acting, radiation-sensitive compositions. More particularly, it relates to negative-acting, radiation-sensitive compositions which can be used in the preparation of imageable elements useful in preparing negative-working lithographic plates, as well as such elements and the lithographic plates prepared therefrom.
The art of lithographic printing depends upon the immiscibility of grease and water, upon the preferential retention of a greasy image-forming substance by an image area, and upon the similar retention of an aqueous dampening fluid by a non-image area. When a greasy image is imprinted upon a suitable surface and the entire surface is then moistened with an aqueous solution, the image area will repel the water and the non-image area will retain the water. Upon subsequent application of greasy ink, the image portion retains the ink whereas the moistened non-image area repels it. The ink on the image area is then transferred to the surface of a material on which the image is to be reproduced, such as paper, cloth and the like, via an intermediary, so-called offset or blanket cylinder, which is necessary to prevent mirror-image printing.
The type of lithographic printing plate to which the present invention is directed is prepared from an imageable element comprising a lithographically suitable support material having aherent to at least one surface thereof a coating comprising a radiation-sensitive composition. Said element is referred to as "presensitized" or "wipe-on" plates depending upon whether the coating is applied to the base material by the manufacturer or by the lithographer, respectively. Depending upon the nature of the radiation-sensitive coating employed, the treated element may be utilized to reproduce directly the image to which it is exposed, in which case it is termed a positive-acting printing plate, or to produce an image complementary to the one to which it is exposed, in which case it is termed a negative-acting printing plate. In either case, the image area of the developed printing plate is oleophilic and the non-image area is hydrophilic.
In the case of a negative-working element which is exposed to radiation through a negative transparency, the radiation-sensitive material, commonly a diazonium compound, is caused to harden and thereby become insoluble in a desensitizing, (i.e., decoating) composition applied to the element, after radiation exposure, for the purpose of removing that part of the radiation-sensitive coating which, because it was protected from the radiation by the negative, was not radiation hardened. The hardened surface of a negative-acting printing plate will be the oleophilic surface compatible with the greasy ink and is called the "image area". The surface from which the non-hardened radiation-sensitive material has been removed by the desensitizer will be, or can be converted to, a hydrophilic surface having little affinity for the greasy ink and is called the "non-image" area.
In U.S. Pat. No. 4,171,974 there are described negative-acting, radiation-sensitive compositions comprising a negative working lithographic photosensitizer and diacrylated (wherein acrylated refers to either acrylic or methacrylic acid esters) bisphenol-A based epoxy resins. U.S. Pat. No. 4,174,307 describes a similar radiation-sensitive composition which are based on diacrylated oligourethanes. Australian Pat. No. 439,241 also describes radiation-sensitive compositions comprising acrylic and methacrylic esters of diepoxy compounds derived from bisphenols and phenol-formaldehyde novolaks of unsaturated functionality up to 3.3.
The above compositions suffer, due to their low unsaturation levels, from, inter alia, short shelf lives and relatively low radiation-sensitivity while lithographic plates prepared from imageable elements comprising said compositions are subject to the disadvantages, inter alia, of poor contrast between image and non-image areas, peeling of the image areas from the base materials, and short press runs.
It has now been found, in accordance with this invention, that negative-acting radiation-sensitive elements may be prepared having long shelf lives and high radiation-sensitivity which are sueful in the preparation of lithographic printing plates of high contrast between image and non-image areas thereof, high adhesion of the image areas to the support materials and long press runs. It has also been found, in accordance with specific embodiments of the invention, that compositions may be prepared which are aqueous-alkali decoatable and/or in the image areas have increased oleophilicity.